[BUGFIX] Fix Livesearch for Editors

Fix searchlevel comparision for editors using pageUID "0" in livesearch,
because "-1" is also a valid searchlevel.

Resolves: #76432
Releases: master, 7.6
Change-Id: I62a2059a7c7f8cf602e452bc0763c6c2cc3fbbe8
Reviewed-on: https://review.typo3.org/48438


Tested-by: Marco Bresch <marco.bresch@starfinanz.de>
Reviewed-by: Dominique Kreemers <dominique.kreemers@icloud.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>